1. Field of the Invention
The present invention relates to a speed control system for a vehicle, particularly a tractor, having a propelling system driven by an engine, the propelling system including a stepless change speed device and an auxiliary change speed device connected in series.
2. Description of the Prior Art
A known speed control system of this type includes a hand accelerator lever for operating a speed governor to control rotational rate of the engine, and a foot pedal for operating a change speed lever connected to the stepless change speed device. Generally, running speed of the vehicle is controlled by depressing the foot pedal with the hand accelerator lever maintained in a selected control position to maintain the rotational rate of the engine constant. The auxiliary change speed device is used to produce high-speed and low-speed states in accordance with working conditions and running conditions. The auxiliary change speed device itself is controlled entirely independently of the controls of the stepless change speed device and the rotational rate of the engine.
The feature that the rotational rate of the engine is maintained constant, regardless of operations of the stepless change speed device and auxiliary change speed device, by maintaining the hand accelerator lever in a selected control position, is very useful in that a working implement such as a rotary plow can be driven at a constant rate despite changes in the vehicle running speed during a field operation. This feature, however, is inconvenient for road running of the vehicle.
That is, for high speed road running, it is necessary to maintain the engine at or adjacent a maximum rotational rate. On the other hand, the vehicle must be stopped and started relatively frequently during road running, such as at intersections and traffic lights. For stopping the vehicle, the stepless change speed device is returned to neutral but this is not sufficient. Although the vehicle stops, the engine keeps rotating at a high rate and producing great noise. It is therefore necessary to return the hand accelerator lever to an idling position, which gives the driver an additional step of operation. For starting the vehicle, the hand accelerator lever must be placed in or adjacent a maximum speed position again, to further increase the number of operating steps.